Chromogranin A. Chromogranin a (cga) is a protein made by certain cells scattered throughout many tissues in the body. These cells are called neuro-endocrine cells, because they behave somewhat like nerve cells, and somewhat like endocrine, or glandular cells. They produce a variety of hormone and Amino Acid derivatives, and can form tumors. High cga levels mean a person might have such a tumor, but isn't certain.